The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Engines may be operated in a spark ignition (SI) mode and a homogeneous charge compression ignition (HCCI) mode. The HCCI mode refers to compressing a mixture of fuel and an oxidizer to a point of auto-ignition. The HCCI and SI modes may be selected based on engine speed and load. In the HCCI mode, ignition occurs at several locations at a time, which makes a fuel/air mixture burn nearly simultaneously.
The HCCI mode performs close to an ideal OTTO cycle, provides improved operating efficiency (operates leaner and with lower pumping losses due less intake air throttling), and generates lower emission levels as compared to lean operation in spark ignition (SI) engines. However since there is no direct initiator of combustion, the ignition process may be more difficult to control.
To adjust operation during the HCCI mode, a control system may alter the conditions that induce combustion. For example, the control system may adjust effective compression ratios, induced gas temperature, induced gas pressure, or the quantity of retained or reinducted exhaust. Several approaches have been used to perform the adjustments and thus extend the HCCI operating region by providing finer control over temperature-pressure-time histories within a combustion chamber.
One control approach employs variable valve timing to adjust the effective compression ratio. For example, the compression ratio can be controlled by adjusting when intake valves open and close. The amount of exhaust gas retained in a combustion chamber can be controlled by valve re-opening and/or valve overlap.
Another approach is referred to as a “2-step” intake and exhaust valve lift approach, which includes switching intake and exhaust valve modes between a HIGH lift state and a LOW lift state. During the HIGH lift state, the intake and exhaust valves are lifted to a HIGH level to allow for an amount of air to enter the corresponding cylinders. During the LOW lift state, the valves are switched to a LOW level, which allows a smaller amount of air to enter the corresponding cylinders relative to the HIGH lift state.
The 2-step approach tends to have inconsistent and non-uniform transitions between SI and HCCI modes. In other words, there may be undesirable torque disturbances during the transitions.